1. Field of the Invention
The present invention relates to polar modulation transmission apparatus, and particularly relates to polar modulation transmission apparatus and wireless communication apparatus employing this apparatus, provided with an amplitude modulation amplifying section amplifying an amplitude modulation signal according to a battery voltage, and applied with an amplitude modulation signal amplified by the amplitude modulation amplifying section as a battery voltage for a high-frequency power amplifier amplifying power of a phase-modulated high-frequency signal, that may be widely applied, for example, to wireless equipment carrying out wireless communication of a polar modulation scheme such as, for example, communication terminals such as mobile telephones, or base stations, etc.
2. Description of the Related Art
In the related art, a polar modulation method is proposed as technology capable of providing both high power efficiency and linearity. The basic configuration of the polar modulation is such that a high-frequency phase modulation signal is inputted to a signal input terminal of a high-frequency power amplifier and a battery voltage of a level adjusted according to an amplitude modulation signal is applied to a battery voltage input terminal of the high-frequency power amplifier. By this means, a high-frequency phase modulation signal inputted to the high-frequency power amplifier can be taken to be a constant-envelope signal that does not have a fluctuating component in an amplitude direction, so that a highly efficient non-linear amplifier can be used as a high-frequency power amplifier.
A configuration disclosed in U.S. Pat. No. 6,377,784 is shown in FIG. 1 as an example configuration for a polar modulation transmission apparatus of the related art.
Polar modulation transmission apparatus 10 inputs transmission data S1 to modulation encoding section 11. Modulation encoding section 11 forms amplitude modulation signal S2 (for example, √(I2+Q2)) expressing the transmission data S1 in polar coordinates and phase modulation signal S3 and transmits amplitude modulation signal S2 to amplitude signal amplifying section 13 and phase modulation signal S3 to carrier wave generation/phase modulation section 12.
Carrier wave generation/phase modulation section 12 is comprised of, for example, a frequency synthesizer, forms phase modulation RF signal S4 by modulating a carrier wave using phase modulation signal S3, and transmits this to a signal input terminal of high-frequency power amplifier 14.
On the other hand, polar modulation transmission apparatus 10 forms amplitude modulation signal S5 constituting a battery voltage applied to a battery terminal of high-frequency power amplifier 14 in the following manner. Battery voltage S7 generated by battery 15 is supplied to DC/DC converter 16. As shown in FIG. 2, DC/DC converter 16 regulates the level of battery voltage S7 according to transmission power control signal S8 for controlling final transmission power for transmissions from polar modulation transmission apparatus 10 and supplies the level-adjusted battery voltage S9 to amplitude signal amplifying section 13. Amplitude signal amplifying section 13 is configured from A linear regulator, amplifies amplitude modulation signal S2 (current amplification) according to the value of battery voltage S9 to obtain amplitude modulation signal S5, and applies this to the battery terminal of high-frequency power amplifier 14.
By this means, phase modulation RF signal S4 and amplitude modulation signal S5 are multiplied to obtain RF output signal S6 from high-frequency power amplifier 14. RF output signal S6 is supplied to the transmission antenna (not shown).
A description will be given now for the reason for providing the DC/DC converter 16 at the polar modulation transmission apparatus 10 of FIG. 1.
First, the case where DC/DC converter 16 is not provided, i.e. the case where battery voltage S7 is provided directly (without exception) to amplitude signal amplifying section 13 from battery 15 will be considered. In this case, the following inconveniences occur.
Power loss occurs at the amplitude signal amplifying section 13 according to the difference in potential between the battery voltage (amplitude modulation signal S5) of high-frequency power amplifier 14 and the battery voltage (battery voltage S9) of amplitude signal amplifying section 13. In particular, in the event that the transmission power level (power level of RF output signal S6) is low, the battery voltage of high-frequency power amplifier 14 becomes low. The difference in potential between battery voltage S7 and amplitude modulation signal S5 is therefore amplified, and, as a result, power loss at the amplitude signal amplifying section 13 becomes large.
Because of this, DC/DC converter 16 is provided in the configuration of FIG. 1 in order to reduce power loss at amplitude signal amplifying section 13. Namely, battery voltage S9 supplied to amplitude signal amplifying section 13 is made to change according to a value (transmission power control signal S8) set for the RF output level of high-frequency power amplifier 14 by DC/DC converter 16. In this way, it is possible to make potential difference between the battery voltage S9 and the amplitude modulation signal S5 small even in cases where the level of the RF output signal of the high-frequency power amplifier 14 is low, so that power loss at amplitude signal amplifying section 13 can therefore be suppressed.
However, the configuration of the related art shown in FIG. 1 does not give sufficient consideration to power loss at DC/DC converter 16.
It is possible to make potential difference between the battery voltage S9 and the amplitude modulation signal S5 small in cases where, for example, transmission power (power level of RF output signal S6) is low, and power loss at amplitude signal amplifying section 13 can therefore be effectively suppressed. On the other hand, in the case that transmission power is high, difference in potential between the battery voltage S7 of amplitude signal amplifying section 13 and amplitude modulation signal S5 is small even when there is no DC/DC converter 16 present and power loss therefore occurs at the DC/DC converter 16 regardless of the fact that power loss at amplitude signal amplifying section 13 is small. Providing of the DC/DC converter 16 therefore conversely increases power loss of the whole of the apparatus.